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Chapter 10
Molecular Biology of the Gene
PowerPoint Lectures
Campbell Biology: Concepts & Connections, Eighth Edition
REECE • TAYLOR • SIMON • DICKEY • HOGAN
© 2015 Pearson Education, Inc.
Lecture by Edward J. Zalisko
THE STRUCTURE OF
THE GENETIC MATERIAL
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10.1 SCIENTIFIC THINKING: Experiments
showed that DNA is the genetic material
• Early in the 20th century, the molecular basis for
inheritance was a mystery.
• Biologists did know that genes were located on
chromosomes. But it was unknown if the genetic
material was
• proteins or
• DNA.
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10.1 SCIENTIFIC THINKING: Experiments
showed that DNA is the genetic material
• Biologists finally established the role of DNA in
heredity through experiments with bacteria and the
viruses that infect them.
• This breakthrough ushered in the field of
molecular biology, the study of heredity at the
molecular level.
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10.1 SCIENTIFIC THINKING: Experiments
showed that DNA is the genetic material
• In 1928, Frederick Griffith was surprised to find
that when he killed pathogenic bacteria, then
mixed the bacterial remains with living harmless
bacteria, some living bacterial cells became
pathogenic.
• All of the descendants of the transformed bacteria
inherited the newly acquired ability to cause
disease.
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10.1 SCIENTIFIC THINKING: Experiments
showed that DNA is the genetic material
• In 1952, Alfred Hershey and Martha Chase used
bacteriophages to show that DNA is the genetic
material of T2, a virus that infects the bacterium
Escherichia coli (E. coli).
• Bacteriophages (or phages for short) are viruses that
infect bacterial cells.
• Phages were labeled with radioactive sulfur to detect
proteins or radioactive phosphorus to detect DNA.
• Bacteria were infected with either type of labeled phage
to determine which substance was injected into cells
and which remained outside the infected cell.
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10.1 SCIENTIFIC THINKING: Experiments
showed that DNA is the genetic material
• The sulfur-labeled protein stayed with the phages
outside the bacterial cell, while the phosphoruslabeled DNA was detected inside cells.
• Cells with phosphorus-labeled DNA produced new
bacteriophages with radioactivity in DNA but not in
protein.
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Figure 10.1a-0
Head
Tail
Tail fiber
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DNA
Figure 10.1b-0
Phage
Bacterium
Radioactive
protein
Empty
protein shell
The radioactivity
is in the liquid.
Phage
DNA
DNA
Centrifuge
Pellet
Batch 1: Radioactive protein labeled in yellow
Radioactive
DNA
Centrifuge
Pellet
Batch 2: Radioactive DNA labeled in green
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The radioactivity
is in the pellet.
10.2 DNA and RNA are polymers of
nucleotides
• DNA and RNA are nucleic acids consisting of long
chains (polymers) of chemical units (monomers) called
nucleotides.
• One of the two strands of DNA is a DNA
polynucleotide, a nucleotide polymer (chain).
• A nucleotide is composed of a
• nitrogenous base,
• five-carbon sugar, and
• phosphate group.
• The nucleotides are joined to one another by a sugarphosphate backbone.
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Figure 10.2a-0
A
C
T
G
A
T
C
Sugar-phosphate
backbone
G
T
A
C
G
C
T
A
C
A
T
C
Covalent
bond
joining
nucleotides
T
A
G
A
A
G
Phosphate
group
Nitrogenous
base
Sugar
Nitrogenous base
(can be A, G, C, or T)
C
G
T
A
A DNA
double helix
DNA
nucleotide
T
G
T
G
Thymine
(T)
Phosphate
group
Sugar
(deoxyribose)
DNA nucleotide
G
G
Two representations
of a DNA polynucleotide
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10.2 DNA and RNA are polymers of
nucleotides
• Each type of DNA nucleotide has a different
nitrogen-containing base:
•
•
•
•
adenine (A),
cytosine (C),
thymine (T), and
guanine (G).
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Figure 10.2b-0
Thymine (T)
Pyrimidines
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Cytosine (C)
Adenine (A)
Guanine (G)
Purines
10.2 DNA and RNA are polymers of
nucleotides
• The full name for DNA is deoxyribonucleic acid,
with nucleic referring to DNA’s location in the
nuclei of eukaryotic cells.
• RNA (ribonucleic acid) is unlike DNA in that it
• uses the sugar ribose (instead of deoxyribose in
DNA) and
• has a nitrogenous base uracil (U) instead of
thymine.
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Figure 10.2c
Nitrogenous base
(can be A, G, C, or U)
Phosphate
group
Uracil (U)
Sugar
(ribose)
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10.3 DNA is a double-stranded helix
• After the 1952 Hershey-Chase experiment
convinced most biologists that DNA was the
material that stored genetic information, a race
was on to determine how the structure of this
molecule could account for its role in heredity.
• Researchers focused on discovering the threedimensional shape of DNA.
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10.3 DNA is a double-stranded helix
• American James D. Watson journeyed to
Cambridge University in England, where the more
senior Francis Crick was studying protein structure
with a technique called X-ray crystallography.
• While visiting the laboratory of Maurice Wilkins at
King’s College in London, Watson saw an X-ray
image of DNA produced by Wilkins’s colleague,
Rosalind Franklin.
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Figure 10.3a-0
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10.3 DNA is a double-stranded helix
• Watson deduced the basic shape of DNA to be a
helix (spiral) with a uniform diameter and the
nitrogenous bases located above one another like
a stack of dinner plates.
• The thickness of the helix suggested that it was
made up of two polynucleotide strands.
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10.3 DNA is a double-stranded helix
• Watson and Crick realized that DNA consisted of
two polynucleotide strands wrapped into a double
helix.
• The sugar-phosphate backbone is on the outside.
• The nitrogenous bases are perpendicular to the
backbone in the interior.
• Specific pairs of bases give the helix a uniform
shape.
• A pairs with T, forming two hydrogen bonds, and
• G pairs with C, forming three hydrogen bonds.
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Figure 10.3b
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Figure 10.3d-0
C
C
G
Hydrogen bond
C
G
G C
G
A
C
Base pair
A
T
G
T
T
C
A
G
A
T
A
T
C
G
G
C
C
G
C
A
A
T
A
G
T
T
T
A
Ribbon model
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Partial chemical structure
Computer model
10.3 DNA is a double-stranded helix
• In 1962, the Nobel Prize was awarded to James D.
Watson, Francis Crick, and Maurice Wilkins.
• Rosalind Franklin probably would have received the
prize as well but for her death from cancer in 1958.
• Nobel Prizes are never awarded posthumously.
• The Watson-Crick model gave new meaning to the
words genes and chromosomes. The genetic
information in a chromosome is encoded in the
nucleotide sequence of DNA.
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DNA REPLICATION
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10.4 DNA replication depends on specific
base pairing
• DNA replication follows a semiconservative
model.
• The two DNA strands separate.
• Each strand then becomes a template for the
assembly of a complementary strand from a supply
of free nucleotides.
• Each new DNA helix has one old strand with one
new strand.
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Figure 10.4a-3
T
T
A
T
A
T
G
C
G
C
G
G
C
G
C
G
C
T
A
T
A
T
A
T
A
T
A
T
A
T
A
A
T
A
C
G
C
G
C
A
T
A parental
molecule of DNA
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G
A
C
Free nucleotides
The parental strands separate
and serve as templates
Two identical daughter
molecules of DNA are formed
Figure 10.4b
A
T
G
C
A
A
T
T
T
A
Parental DNA
molecule
Daughter
strand Parental
strand
Daughter DNA
molecules
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10.5 DNA replication proceeds in two
directions at many sites simultaneously
• Replication of a DNA molecule begins at particular
sites called origins of replication, short stretches of
DNA having a specific sequence of nucleotides.
• Proteins that initiate DNA replication
• attach to the DNA at the origin of replication and
• separate the two strands of the double helix.
• Replication then proceeds in both directions,
creating replication “bubbles.”
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10.5 DNA replication proceeds in two
directions at many sites simultaneously
• DNA replication occurs in the 5 to 3 direction.
• Replication is continuous on the 3 to 5 template.
• DNA polymerases add nucleotides only to the 3
end of the strand, never to the 5 end.
• Replication is discontinuous on the 5 to 3
template, forming short Okazaki fragments.
• An enzyme, called DNA ligase, links (or ligates)
the pieces together into a single DNA strand.
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Figure 10.5b
5′ end
P
3′ end
HO
5′
4′
3′
2′
1′
2′
A
T
5′
P
C
P
G
C
P
P
T
3′ end
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P
G
P
OH
3′
4′
1′
A
P
5′ end
Figure 10.5c
DNA polymerase
molecule
5′
3′
Parental DNA
Replication fork
5′
3′
DNA ligase
Overall direction of replication
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3′
5′
This daughter
strand is
synthesized
continuously
This daughter
strand is
3′ synthesized
5′ in pieces
10.5 DNA replication proceeds in two
directions at many sites simultaneously
• DNA polymerases and DNA ligase also repair DNA
damaged by harmful radiation and toxic chemicals.
• DNA replication ensures that all the somatic cells
in a multicellular organism carry the same genetic
information.
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THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
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10.6 Genes control phenotypic traits through
the expression of proteins
• DNA specifies traits by dictating protein synthesis.
• Proteins are the links between genotype and
phenotype.
• The molecular chain of command is from DNA in
the nucleus to RNA and RNA in the cytoplasm to
protein.
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10.6 Genes control phenotypic traits through
the expression of proteins
• Transcription is the synthesis of RNA under the
direction of DNA.
• Translation is the synthesis of proteins under the
direction of RNA.
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Figure 10.6a-3
DNA
Transcription
RNA
NUCLEUS
Translation
Protein
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CYTOPLASM
10.6 Genes control phenotypic traits through
the expression of proteins
• Genes provide the instructions for making specific
proteins.
• The initial one gene–one enzyme hypothesis was
based on studies of inherited metabolic diseases.
• The one gene–one enzyme hypothesis was
expanded to include all proteins.
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10.6 Genes control phenotypic traits through
the expression of proteins
• Most recently, the one gene–one polypeptide
hypothesis recognizes that some proteins are
composed of multiple polypeptides.
• Even this description is not entirely accurate, in
that the RNA transcribed from some genes is not
translated but nonetheless has important functions.
• In addition, many eukaryotic genes code for a set
of polypeptides (rather than just one) by a process
called alternative splicing.
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10.7 Genetic information written in codons is
translated into amino acid sequences
• The sequence of nucleotides in DNA provides a
code for constructing a protein.
• Protein construction requires a conversion of a
nucleotide sequence to an amino acid sequence.
• Transcription rewrites the DNA code into RNA,
using the same nucleotide “language.”
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10.7 Genetic information written in codons is
translated into amino acid sequences
• The flow of information from gene to protein is based
on a triplet code.
• The genetic instructions for the amino acid sequence
of a polypeptide chain are written in DNA and RNA as
a series of nonoverlapping three-base “words” called
codons.
• Translation involves switching from the nucleotide
“language” to the amino acid “language.”
• Each amino acid is specified by a codon.
• 64 codons are possible.
• Some amino acids have more than one possible codon.
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Figure 10.7-1
DNA
A
A
A
C
C
G
G
C
A
A
A
A
U
U
U
G
G
C C
G
U
U
U
U
Transcription
RNA
Translation
Codon
Polypeptide
Amino acid
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10.8 The genetic code dictates how codons
are translated into amino acids
• The genetic code is the amino acid translations of
each of the nucleotide triplets.
• Three nucleotides specify one amino acid.
• Sixty-one codons correspond to amino acids.
• AUG codes for methionine and signals the start of
transcription.
• Three “stop” codons signal the end of translation.
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10.8 The genetic code dictates how codons
are translated into amino acids
• The genetic code is
• redundant, with more than one codon for some
amino acids,
• unambiguous, in that any codon for one amino acid
does not code for any other amino acid, and
• nearly universal, in that the genetic code is shared
by organisms from the simplest bacteria to the most
complex plants and animals.
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Figure 10.8a
Second base of RNA codon
C
A
UUU
U
UUC
First base of RNA codon
UUA
C
A
Leu
UCU
UAU
UCC
UAC
UCA
Ser
Tyr
UGU
UGC
Cys
U
C
UAA Stop UGA Stop A
UUG
UCG
UAG Stop UGG Trp
G
CUU
CCU
CAU
U
CUC
CCC
CAC
Leu
Pro
CAA
His
CGU
CGC
CGA
CUA
CCA
CUG
CCG
CAG
CGG
AUU
ACU
AAU
AGU
ACC
AAC
AUC lle
AUA
G
ACA
Thr
Asn
AAA
AGA
GUU
GAU
GUC
GUG
GCC
Val
GCA
GCG
GAC
Ala
GAA
GAG
Ser
Glu
GGC
GGA
GGG
A
U
C
Arg
A
G
GGU
Asp
C
G
AGC
AAG Lys AGG
GCU
Arg
Gln
or
AUG Met
ACG
start
GUA
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Phe
G
U
Gly
C
A
G
Third base of RNA codon
U
Figure 10.8b-1
Strand to be transcribed
T
A C T
T
C A
A
A A
T
C
A
G T
T
T T
A
G
DNA
A T
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G A
Figure 10.8b-2
Strand to be transcribed
T
A C T
T
C A
A
A A
T
C
G A
A
G T
T
T T
A
G
A U G A
A
G U
U
U U
A
G
DNA
A T
Transcription
RNA
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Figure 10.8b-3
Strand to be transcribed
T
A C T
T
C A
A
A A
T
C
G A
A
G T
T
T T
A
G
A U G A
A
G U
U
U U
A
G
DNA
A T
Transcription
RNA
Translation
Start
codon
Polypeptide
Met
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Stop
codon
Lys
Phe
10.9 VISUALIZING THE CONCEPT:
Transcription produces genetic messages in
the form of RNA
• Transcription of a gene occurs in three main steps:
1. initiation, involving the attachment of RNA
polymerase to the promoter and the start of
RNA
synthesis,
2. elongation, as the newly formed RNA strand
grows, and
3. termination, when RNA polymerase reaches the
terminator DNA and the polymerase
molecule
detaches from the newly made RNA strand
and
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Figure 10.9-3
Direction of transcription
Initiation
RNA synthesis begins after RNA
polymerase attaches to the promoter.
Unused
strand
of DNA
RNA polymerase
Terminator
DNA
DNA
of gene
Newly formed
RNA
Promoter
Elongation
Template strand
of DNA
Direction of transcription
Using the DNA as a template, RNA
polymerase adds free RNA nucleotides
one at a time.
Free RNA
nucleotide
DNA strands
reunite
T C C A A
U C C A
A GG T T
DNA strands
separate
Newly made RNA
Termination
RNA synthesis ends when RNA
polymerase reaches the
terminator DNA sequence.
Terminator
DNA
Completed RNA
RNA polymerase
detaches
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10.10 Eukaryotic RNA is processed before
leaving the nucleus as mRNA
• Messenger RNA (mRNA)
• encodes amino acid sequences and
• conveys genetic messages from DNA to the
translation machinery of the cell.
• In prokaryotes, this occurs in the same place that
mRNA is made.
• But in eukaryotes, mRNA must exit the nucleus via
nuclear pores to enter the cytoplasm.
• Eukaryotic mRNA has introns, interrupting
sequences that separate exons, the coding
regions.
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10.10 Eukaryotic RNA is processed before
leaving the nucleus as mRNA
• Eukaryotic mRNA undergoes processing before
leaving the nucleus.
• RNA splicing removes introns (intervening
sequences) and joins exons (expressed
sequences) to produce a continuous coding
sequence.
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10.10 Eukaryotic RNA is processed before
leaving the nucleus as mRNA
• A cap and tail of extra nucleotides are added to the
ends of the mRNA to
• facilitate the export of the mRNA from the nucleus,
• protect the mRNA from degradation by cellular
enzymes, and
• help ribosomes bind to the mRNA.
• The cap and tail themselves are not translated into
protein.
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Figure 10.10
Exon
DNA
Exon
Intron
Cap
RNA
transcript
with cap
and tail
Exon
Intron
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
NUCLEUS
CYTOPLASM
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10.11 Transfer RNA molecules serve as
interpreters during translation
• Transfer RNA (tRNA) molecules function as an
interpreter, converting the genetic message of
mRNA into the language of proteins.
• Transfer RNA molecules perform this interpreter
task by
• picking up the appropriate amino acid and
• using a special triplet of bases, called an
anticodon, to recognize the appropriate codons in
the mRNA.
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10.12 Ribosomes build polypeptides
• Translation occurs on the surface of the ribosome.
• Ribosomes coordinate the functioning of mRNA and
tRNA and, ultimately, the synthesis of polypeptides.
• Ribosomes have two subunits: small and large.
• Each subunit is composed of ribosomal RNAs and
proteins.
• Ribosomal subunits come together during
translation.
• Ribosomes have binding sites for mRNA and
tRNAs.
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Figure 10.12-0
tRNA
molecules
tRNA binding sites
Growing
polypeptide
Ribosome
Large
subunit
P A
site site
Small
subunit
mRNA binding site
The next amino
acid to be added
to the polypeptide
Growing
polypeptide
mRNA
tRNA
Codons
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10.12 Ribosomes build polypeptides
• The ribosomes of bacteria and eukaryotes are very
similar in function.
• Those of eukaryotes are slightly larger and
different in composition.
• The differences are medically significant.
• Certain antibiotic drugs can inactivate bacterial
ribosomes while leaving eukaryotic ribosomes
unaffected.
• These drugs, such as tetracycline and
streptomycin, are used to combat bacterial
infections.
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10.13 An initiation codon marks the start of
an mRNA message
• Translation can be divided into the same three
phases as transcription:
1. initiation,
2. elongation, and
3. termination.
• Initiation brings together
• mRNA,
• a tRNA bearing the first amino acid, and
• the two subunits of a ribosome.
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10.13 An initiation codon marks the start of
an mRNA message
• Initiation establishes where translation will begin.
• Initiation occurs in two steps.
1. An mRNA molecule binds to a small ribosomal
subunit, and a special initiator tRNA binds to
mRNA at the start codon.
• The start codon reads AUG and codes for
methionine.
• The first tRNA has the anticodon UAC.
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10.13 An initiation codon marks the start of
an mRNA message
• Initiation establishes where translation will begin.
• Initiation occurs in two steps.
2. A large ribosomal subunit joins the small subunit,
allowing the ribosome to function.
• The first tRNA occupies the P site, which will hold
the growing polypeptide.
• The A site is available to receive the next aminoacid-bearing tRNA.
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Figure 10.13a
Start of genetic message
Cap
End
Tail
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Figure 10.13b-1
Initiator
tRNA
mRNA
U A C
A U G
Start codon
1
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Small ribosomal
subunit
Figure 10.13b-2
Large
ribosomal
subunit
Initiator
tRNA
mRNA
P
site
A
site
U A C
U A C
A U G
A U G
Start codon
1
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Small ribosomal
subunit
2
10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon
terminates translation
• Once initiation is complete, amino acids are added
one by one to the first amino acid.
• Each addition occurs in a three-step elongation
process.
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10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon
terminates translation
• Each cycle of elongation has three steps.
1. The anticodon of an incoming tRNA molecule,
carrying its amino acid, pairs with the mRNA
codon in the A site of the ribosome.
2. The polypeptide separates from the tRNA in the P
site and attaches by a new peptide bond to the
amino acid carried by the tRNA in the A site.
3. The P site tRNA (now lacking an amino acid)
leaves the ribosome, and the ribosome
translocates (moves) the remaining tRNA (which
has the growing polypeptide) from the A site to
the P site.
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Figure 10.14-4
Amino
acid
Anticodon
A site
Polypeptide
P
site
mRNA
Codons
1 Codon
recognition
mRNA
movement
Stop
codon
New
peptide
bond
3
Translocation
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2 Peptide bond
formation
10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon
terminates translation
• Elongation continues until the termination stage of
translation, when
• the ribosome reaches a stop codon,
• the completed polypeptide is freed from the last
tRNA, and
• the ribosome splits back into its separate subunits.
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10.15 Review: The flow of genetic information
in the cell is DNA  RNA  protein
• The flow of genetic information is from DNA to
RNA to protein.
• In transcription (DNA → RNA), the mRNA is
synthesized on a DNA template.
• In eukaryotic cells, transcription occurs in the
nucleus, and the messenger RNA is processed
before it travels to the cytoplasm.
• In prokaryotes, transcription occurs in the
cytoplasm.
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10.15 Review: The flow of genetic information
in the cell is DNA  RNA  protein
• Translation can be divided into four steps, all of
which occur in the cytoplasm:
1.
2.
3.
4.
amino acid attachment,
initiation of polypeptide synthesis,
elongation, and
termination.
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Figure 10.15-5
DNA
Transcription
NUCLEUS
mRNA
Transcription
1
RNA
polymerase
CYTOPLASM
Translation
Amino acid
Amino acid
attachment
Enzyme
2
tRNA
Initiator
tRNA
UA C
AU G
mRNA
Start
codon
ATP
Large
ribosomal
subunit
Anticodon
3 Initiation of
polypeptide
synthesis
Small
ribosomal
subunit
New peptide
bond forming
Growing
polypeptide
4
Elongation
Codons
mRNA
Polypeptide
5
Stop codon
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Termination
10.16 Mutations can affect genes
• A mutation is any change in the nucleotide
sequence of DNA.
• Mutations can involve
• large chromosomal regions or
• just a single nucleotide pair.
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10.16 Mutations can affect genes
• Mutations within a gene can be divided into two
general categories.
1. Nucleotide substitutions involve the replacement of
one nucleotide and its base-pairing partner with
another pair of nucleotides. Base substitutions may
• have no effect at all, producing a silent mutation,
• change the amino acid coding, producing a missense
mutation, which produces a different amino acid,
• lead to a base substitution that produces an improved
protein that enhances the success of the mutant
organism and its descendants, or
• change an amino acid into a stop codon, producing a
nonsense mutation.
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10.16 Mutations can affect genes
2. Nucleotide insertions or deletions of one or more
nucleotides in a gene may
• cause a frameshift mutation, which alters the
reading frame (triplet grouping) of the genetic
message,
• lead to significant changes in amino acid sequence,
and
• produce a nonfunctional polypeptide.
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10.16 Mutations can affect genes
• Mutagenesis is the production of mutations.
• Mutations can be caused
• by spontaneous errors that occur during DNA
replication or recombination or
• by mutagens, which include
• high-energy radiation such as X-rays and ultraviolet
light and
• chemicals.
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Figure 10.16a
Normal hemoglobin DNA
Mutant hemoglobin DNA
C T T
C A T
mRNA
mRNA
G A A
G U A
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
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Figure 10.16b-0
Normal
gene
A U G A A G U U U G G C G C A
mRNA
Lys
Phe
Gly
Ala
Protein Met
Nucleotide
substitution
A U G A A G U U U A G C G C A
Met
Lys
Phe
Ser
Ala
Deleted
Nucleotide
deletion
A U G A A G U U G G C G C A
Met
Lys
Leu
Ala
Inserted
Nucleotide
insertion
A U G A A G U U
Met
© 2015 Pearson Education, Inc.
Lys
Leu
U G G C G C
Trp
Arg
Figure 10.16b-1
Normal
gene
mRNA
Protein
Nucleotide
substitution
A U G A A G U U U G G C G C A
Met
Phe
Gly
Ala
A U G A A G U U U A G C G C A
Met
© 2015 Pearson Education, Inc.
Lys
Lys
Phe
Ser
Ala
Figure 10.16b-2
Normal
gene
mRNA
Protein
A U G A A G U U U G G C G C A
Met
Lys
Phe
Gly
Ala
Deleted
Nucleotide
deletion
A U G A A G U U G G C G C A
Met
© 2015 Pearson Education, Inc.
Lys
Leu
Ala
Figure 10.16b-3
Normal
gene
mRNA
Protein
A U G A A G U U U G G C G C A
Met
Lys
Phe
Gly
Ala
Inserted
Nucleotide
insertion
A U G A A G U U
Met
© 2015 Pearson Education, Inc.
Lys
Leu
U G G C G C
Trp
Arg